1. Field of the Invention
This invention relates to an analog optical transmission system and an optical fiber amplifier.
2. Description of the Prior Art
An analog optical transmission system is known. Such an analog optical transmission system transmits a video signal such that laser light is intensity-modulated with the video signal directly and the modulated laser light is transmitted through an optical fiber. In such an analog optical transmission system, a plurality of channels of video signals may be transmitted by frequency multiplexing techniques. This prior art analog optical transmission system features a low cost per channel in building transmission lines than other systems. Moreover, if the system is used in a CATV network, the number of relay amplifiers is reduced because, in this system, a transmission distance without a relay can be larger than in coaxial cable transmission line systems.
FIG. 9 is a block diagram of a prior art analog optical transmission system. The prior art analog optical transmission system comprises: an amplifier 2 to which a video signal is applied through an input terminal 1; a semiconductor laser module 3 for emitting laser light and for intensity-modulating the laser light with the amplified video signal; an optical fiber 4 for transmitting the laser light emitted and modulated by the laser module 3; a photosensitive unit 5 for receiving the transmitted laser light and converting it into an electronic signal; and an amplifier 6 for amplifying the electronic signal to output the amplified electronic signal at an output terminal 7. The semiconductor laser modulator 3 is connected to the optical fiber 4 by an optical connector 8. Similarly, the photosensitive unit 5 is connected to the optical fiber 4 by an optical connector 9. A plurality of analog video signals (channel 1 to channel N; N is a natural number more than one) are RF-multiplexed and are added to each other to produce RF-multiplexed analog video signals CH1-CHN. The RF-multiplexed analog video signals are inputted to an input terminal 1 to transmit the video signals to the photosensitive unit 5. The laser module 3 intensity-modulates an output laser beam in accordance with the inputted RF-multiplexed analog video signals. This prior art analog optical transmission system can transmit analog AM (amplitude modulation) video signals of more than 40 channels over 10 Km distance without a relay.
Optical fiber amplifiers are known in the art. Such an optical fiber amplifier comprises a quartz type optical fiber having a core into which a rare earth element, such as erbium (Er), is doped. The erbium-doped optical fiber exhibits amplification properties in the 1550 nm band where quartz type optical fibers show low loss. Such an optical fiber amplifier improves allowable dissipation if it is used as an in-line amplifier or a post amplifier.
However, in the prior art analog optical transmission system shown in FIG. 9, there is a problem that noise or distortion of the received video signal at the output terminal 7 is developed by multi-reflection between the optical connectors 8 and 9. More specifically, wavelength variation of the laser light due to modulation with the video signals is converted into magnitude variation by the multi-reflection between the optical connectors 8 and 9, so that noise or distortion is developed due to the magnitude variation in the received video signal at the output terminal 7.
Hereinbelow will be described this problem concretely with reference to FIG. 10. FIG. 10 is a spectrum diagram of the received video signals in the prior art analog optical transmitted system shown in FIG. 9.
In this prior art analog optical transmission system, a modulation frequency is 300 MHz (RF band) and a modulation factor is 5%. As shown in FIG. 10, in the prior art analog optical transmission system, large peaks of interference noise are developed at a lower frequency region than the modulation frequency 300 MHz and harmonic frequencies (600 MHz and 900 MHz) of the modulation frequency. The shape of the noise peaks reflects a spectrum width of the laser module 3 and frequency bands of the noise peaks are dependent on an amount of a maximum frequency shift of the laser light.
In the optical fiber amplifier, the larger the amount of multi-reflection laser light, the larger in the magnitude of interference noise developed in the optical transmission line. Therefore, in an analog optical transmission system in which an optical fiber amplifier is used, an S/N ratio decreases when the multi-reflection occurs in the optical fiber amplifier. This is because multi-reflected light is also amplified, so that interference noise is increased.